1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device in the FFS (Fringe Field Switching) mode.
2. Description of Related Art
A liquid crystal display device is one of flat panels, and it is widely used in monitors of personal computers and portable information terminal devices, etc., by taking advantage of its low power consumption, compactness, and lightweight. In addition, the liquid crystal display device is also widely used for TV sets, and it is replacing conventional cathode-ray tubes.
A mainstream of the liquid crystal display device in recent years is an active matrix type in which a plurality of display signal wires and a plurality of scanning signal wires are arranged in a lattice, and in which a thin film transistor (hereinafter also referred to as “TFT” (Thin Film Transistor)) is formed as a switching element in a pixel area surrounded with the display signal wires and the scanning signal wires. A structure and a material of the TFT are arbitrarily selected according to an application and a required performance of the display device. As for a structure of the TFT, an MOS (Metal Oxide Semiconductor) structure, such as a bottom gate type (reverse stagger type) and a top gate type (stagger type), is often employed. There are included an amorphous silicon film, a polycrystalline silicon (polysilicon) film, etc as a semiconductor film that constitutes the TFT.
A TFT using the polycrystalline silicon film as a channel active layer has high electron mobility. A performance of the active matrix type display device has been rapidly improved by utilizing the polycrystalline silicon film. The TFT using the polycrystalline silicon film is used for forming a circuit around the display device, whereby use of an IC and an IC-mounted substrate can be reduced. This allows to simplify a configuration of the display device to thereby achieve reduction in size thereof, and also allows to enhance reliability thereof.
Meanwhile, development of the multimedia industry in recent years has strongly required a high-definition image display device. A conventional liquid crystal driving system, TN (Twisted Nematic) mode, is a system that changes a display state by applying a vertical electric field perpendicular to a substrate, raising or laying liquid crystal molecules with respect to a substrate surface according to a condition of applying a voltage, and it has a poor viewing angle characteristic due to a principle thereof. An IPS (In-Plane Switching) mode is a system that moves the liquid crystal molecules within a surface parallel to the substrate to thereby change the display state ON or OFF by applying a horizontal electric field parallel to the substrate. The system is then widely accepted in the market since a change of retardation, which is a phase difference of a liquid crystal layer due to viewing angles, is small, and the viewing angle characteristic is wide.
In recent years, the FFS mode has been developed by further improving the IPS mode (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-182230). The FFS mode is also a system that moves the liquid crystal molecules by mainly applying the horizontal electric field parallel to the substrate, and it is superior in the viewing angle characteristic. A difference between the FFS mode and the IPS mode is the following. Namely, in the IPS mode, a distance between a pixel electrode and a common electrode for driving liquid crystals is larger than a cell gap or an electrode width, while in the FFS mode, the distance between the pixel electrode and the common electrode is smaller than the cell gap or the electrode width. In addition, another difference between the two modes is the following. In the IPS mode, the pixel electrode and the common electrode are arranged not to overlap with each other as viewed from a plane, while in the FFS mode, a plate-shaped electrode and a slit electrode are arranged so as to overlap with each other through an insulator layer above the plate-shaped electrode that constitutes the pixel electrode or the common electrode, the slit electrode constituting either of the pixel electrode or the common electrode (the common electrode when the plate-shaped electrode is the pixel electrode, and the pixel electrode when the plate-shaped electrode is the common electrode).
We have now discovered that in the IPS mode, the liquid crystal molecules located between the pixel electrode and the common electrode are driven as viewed from a plane, while the liquid crystal molecules located above the respective electrodes are hardly driven. For this reason, upper portions of the respective electrodes cannot serve to display, which prevents an aperture ratio from becoming high. Meanwhile, in the case of the FFS mode, not only the liquid crystal molecules located between the pixel electrode and the common electrode but also the ones located above the respective electrodes can be driven. For this reason, if each electrode is formed of a transparent conductive film, such as ITO (Indium Tin Oxide), upper portions of the electrodes can also serve to display. Hence, in a liquid crystal display panel with a similar pixel size, the FFS mode can achieve a higher aperture ratio than the IPS mode.